The present invention relates to a film forming apparatus and a method for determining film thickness control conditions used in the apparatus. The film forming apparatus is used for forming a film of a desired thickness by discharging a process liquid toward a rotating substrate such as optical disk substrates, liquid crystal substrates, and semiconductor substrates, while moving the nozzle, e.g., from an inner radius side to an outer radius side of the rotating substrate.
Conventionally, for the formation of a resist film or other film on a liquid crystal substrate, optical disk substrate, semiconductor substrate, or other substrates by applying thereto a process liquid, such as photoresist, there have been used film forming apparatuses in which the substrate is rotated during the forming of the film.
In this kind of film forming apparatus, there is known an apparatus which comprises a substrate holder for rotating and holding the substrate, a process liquid discharger having a nozzle for discharging the process liquid to the substrate held by the substrate holder, and a process liquid feeder for feeding the process liquid to the process liquid discharger. In order for this film forming apparatus to attain an uniform film thickness, the process liquid needs to be dispensed at the most constant possible rate through the nozzle. For this purpose, the process liquid feeder is arranged so as to pressurize the process liquid at a constant pressure when feeding it to the process liquid discharger.
With this conventional arrangement, since temperature variations of the process liquid causes viscosity variations, the process liquid, even if fed at a constant pressure to the process liquid discharger, might not be discharged at a constant quantity from the nozzle of the process liquid discharger. Pressure variations due to the liquid level being lowered by the consumption of the process liquid may also hinder the process liquid from being discharged at a constant quantity from the process liquid discharger. In such cases, the conventional arrangement has a drawback in that the discharge velocity of the process liquid from the nozzle may vary with variations in the state of the process liquid.
In this film forming apparatus, since the process liquid is applied with the substrate being rotated, the application area of the process liquid increases more rapidly towards the peripheral side of the substrate. Accordingly, in order to attain a uniform film thickness, such control is implemented such that as the discharge head moves toward the peripheral side, the moving speed of the discharge head or the rotational speed of the substrate is decreased, or the flow velocity of discharge from the nozzle is gradually increased.
For this control, control conditions necessary to obtain a desired film thickness, such as the head moving speed or discharge flow velocity or number of revolutions of the substrate, that vary among different radial positions and such control conditions have conventionally been determined by executing actual application processes under the control conditions obtained by rule of thumb and by measuring the resulting film thickness using trial and techniques.
With this conventional arrangement, since the control conditions for forming a desired film thickness are determined by executing actual applications and by measuring the resulting film thicknesses, it would take long time to determine the control conditions. Also, since the control conditions are determined by actual measurements, the determined control conditions are not necessarily be optimum ones, such that the limits for the film thickness control could not be defined. For example, in the case where the control conditions are determined by trial and error, when the resulting film thickness is different from the desired film thickness by 1% or so, it could not be decided whether the resultant film thickness is the limit of control, or it could be made even closer to the desired film thickness.